Generally, this invention relates to an ice sensor or detector for an ice bath-type of heat exchanger, and more particularly to a so-called differential ice sensor for an ice bath-type cold drink or beverage dispenser.
Ice bath cold drink dispensers are well known. An example of such a prior art ice bank beverage despenser is shown in U.S. Pat. No. 3,056,273. Typically, such cold drink dispensers have a refrigeration system having an evaporator or other refrigerated surface immersed in a liquid water bath. The refrigeration system is operated so as to direct refrigerant through the evaporator (or refrigerated surface) thereby to freeze a quantity of ice on the refrigerated surface within the ice bath. By circulating the remaining liquid water in the ice bath around the body of ice and over the beverage flow path, the temperature of the remaining water in the ice bath can be maintained in a substantially isothermal condition at or only slightly above the freezing point. The beverage line or flow path is in direct heat transfer relation with the liquid water in the ice bath such that efficient cooling of the beverage is effected and such that the beverage may be chilled to near the freezing point, without danger of freeze-up of the beverage in the beverage flow path.
During normal usage, the heat given off by the beverage flowing through the beverage line immersed in ice bath water causes the outer surface of the ice body to melt. During normal usage rates, the refrigeration system can make up for the melting of the ice so as to maintain the ice body at a pre-determined size thereby to provide sufficient reserve cooling capacity for peak usage periods. During peak usage periods, the refrigeration system may not be able to remove the heat from the ice bath so as to maintain the body of ice at its pre-determined size. As a result, during such peak usage periods, the size of the ice body may decrease. However, because of the isothermal relationship within the ice bath, the beverage will still be chilled to at or near the freezing point. After the peak usage period has passed, the refrigeration system operates to re-freeze the water such that the ice body will again attain its pre-determined size.
In this manner, a smaller, more efficient and less costly refrigeration system may be utilized for the cold drink dispenser, and yet during peak usage periods, the cold drinks will be dispensed at a chilled temperature at or near the freezing point without chance of freeze-up.
Conventionally, such ice bath cold drink dispensers (and other similar refrigeration systems, such as milk coolers or the like) have utilized sensors to determine when the size of the ice body formed by the refrigerated surface has been frozen or "grown" to a pre-determined size or envelope. When the ice body has attained its predetermined size, the refrigeration system is shut down. As the ice begins to melt, either as the result to beginning to warm to room temperature or as a result of a beverage being chilled by the ice bath, the ice sensor again energizes the refrigeration system to begin replenishment of the ice body to its desired predetermined size.
Reference may be made to such U.S Pat. Nos. as 2,506,775, 3,252,420, 3,496,733, 3,502,899, 4,480,441 and 4,497,179 which disclose a variety of prior art ice and liquid level sensors in the same general field as the present invention.
Recently, an improved cold drink dispenser has been commercially introduced which utilizes a pre-chilling coil to pre cool the incoming beverage (i.e., city tap water to be carbonated in a post mix cold drink dispenser) prior to the beverage being fully cooled in an ice bath chiller. The pre-chilling coil and the ice bath coil are both supplied refrigerant from a common refrigeration system, as required, and as determined by an electronic control system. This two cooling coil cold drink dispenser is described in U.S. Pat. No. 4,754,609 invented by William J. Black and assigned to the Cornelius Company of Anoka, Minn. An improved control system for such a two cooling coil cold drink dispenser is disclosed in U.S. patent application Ser. No. 171,455 invented by David P. Forsythe and co-assigned to the Emerson Electric Co., the assignee of the present application.
Generally, prior art ice body sensors utilized two impedance or conductivity probes positioned in the water such that one of the probes was to sense a position of a minimum size for the ice body and the other was positioned to sense a maximum size for the ice body. However, such prior art ice sensors sensed on the absolute conductivity or impedance of the water such that if the impedance of the water changed sufficiently, as due to changes in dissolved minerals in the water or due to other contamination, false readings of the presence of ice may be detected.